The nature of a process corrupting EMI scanner data was discovered by analysis of representative listings. The process involved spurious cyclic application of logical OR and NOT operations to the underlying bits of the binary data. The errors occurred in a contiguous 6-bit sequence in every consecutive 40-bit string in the files. Relying on an understanding of the corruption process and limiting values of the data, a method was devised by which the data could be restored sufficiently for clinical use. A lengthy and critical reconstruction was made of existing corrupted data.